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technical article
Classification and mechanism of electromagnetic flowmeter
2012-2-13 10:55:41

的安装电磁流量度的正确安装对电磁流量计的正常运行极为重要,这里主要介绍电磁流量传感器和转换器的安装。 Installation of electromagnetic flowmeter The correct installation of the electromagnetic flow rate is extremely important for the normal operation of the electromagnetic flowmeter. Here we mainly introduce the installation of the electromagnetic flow sensor and converter. Electromagnetic flowmeter sensor installation electromagnetic

Pay attention to the following issues when installing the flowmeter sensor:


First, the installation site of the ordinary electromagnetic flow sensor enclosure protection level is IP65 (GB4208 dust and splash-proof water level), the requirements for the installation site are:

1) When measuring mixed-phase fluid, choose a place that will not cause phase separation;


2) Select a place where no negative pressure will appear in the measuring tube;


3) Avoid installing near electric equipment such as electric motors and transformers to avoid electromagnetic field interference;

4) Avoid installing in places with strong corrosive gas;
5) The ambient temperature should generally be in the range of -25 ~ 60 ℃, and avoid direct sunlight as much as possible;
6) Install in a place without vibration or small vibration. If the vibration is too large, fixed support should be added to the pipe before and after the sensor.
7) The relative humidity of the environment should generally be in the range of 10% ~ 90%;
8) Avoid installation in a place where it can be directly exposed to rain or submerged by water. If the sensor's enclosure protection level is IP67 (dust-proof and water-proof) or IP68 (dust-proof and diving-proof), the last two items may not be required.
Second, the length of the straight pipe section The electromagnetic flowmeter has a lower requirement for the length of the straight pipe section before the meter. Generally, for 90 ° elbow, T-shaped tee, reducer, fully open valve and other flow resistance parts, it should have a straight pipe length of 3-5D from the center line of the sensor electrode axis (not the sensor inlet end face); for different openings 10-degree valve requires a straight pipe length of 10D; generally 2D straight pipe length behind the sensor. When the valve cannot be fully opened, if the valve's interception direction is installed at 45 ° with the sensor electrode axis, the additional error can be greatly reduced.
3. Installation position and flow direction The electromagnetic flow sensor can be installed horizontally, vertically or obliquely. For horizontal installation, the sensor electrode shaft must be placed horizontally.
This can prevent the electrodes from being insulated for a short time due to the air bubbles entrained in the fluid; it can also prevent the electrodes from being covered by the deposits in the fluid. The sensor should not be installed at the highest point to avoid accumulation of air. It is installed at the highest point of the piping system. It is a bad installation position and should be avoided.
When installed vertically, the flow direction should be upward, so that no flow or very low flow, the heavy solid particles entrained in the fluid will sink, and the lighter fatty substances will rise away from the sensor electrode area. When measuring liquid-solid two-phase flows such as mud and ore slurry, vertical installation can avoid solid phase sensation precipitation and uneven wear of the sensor lining. At the exit of a downward pipe, a bad installation position should be avoided.
There should be a certain back pressure at the installation place of the sensor. The direct exhaustion of the sensor outlet may easily cause the liquid in the measuring tube to be not full. This is a bad installation position and should be avoided.
To prevent negative pressure in the sensor, the sensor should be installed behind the pump and not in front of the pump.
Fourth, install the bypass pipe In order to facilitate the inspection and adjustment of zero point when the liquid flow is stationary, small and medium pipe diameter should be installed as much as possible. When measuring fluids containing deposits, consider mounting methods that facilitate cleaning of the sensor.
V. Grounding In order to make the measurement accurate and the electrode will not be corroded by current, the electromagnetic flow sensor must be grounded separately, and the sensor and the fluid should be at about the same potential. In principle, the separation type electromagnetic flowmeter should be grounded on the sensor side, and the converter should be grounded on the same ground point. In most cases, the built-in reference electrode or metal tube of the sensor can ensure the potential balance. Therefore, the fluid in the tube can be grounded through the built-in reference electrode and metal tube, and the ground piece of the sensor can be connected to the ground wire.
If the sensor is installed on a pipeline with cathodic corrosion protection, in addition to grounding the sensor and the grounding ring together, a thick copper wire should also be used to bypass the sensor across the two flanges of the pipeline to make the cathodic protection current between the sensor and the sensor.

的基本原理是法拉第电磁感应定律,即导体在磁场中切割磁力线运动时在其两端产生感应电动势。 Principle and mechanism The basic principle of the electromagnetic flowmeter is the Faraday's law of electromagnetic induction, that is, when a conductor cuts magnetic lines of force in a magnetic field, an induced electromotive force is generated at both ends. As shown in Figure 1, a conductive liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, and an induced potential proportional to the flow rate is generated in a direction perpendicular to the flow direction. The direction of the electromotive force is according to the "Fleming right-hand rule" and its value In the following formula, E------ induced electromotive force, that is, the flow signal, V;
k ------- coefficient;
B----magnetic induction, T;
D --- --- measuring tube inner diameter, m;
----- average velocity, m / s.
Let the volume flow of the liquid be, where K is the meter constant, and K = 4 KB / πD.
The electromagnetic flowmeter is composed of a flow sensor and a converter. The typical structure of the sensor is shown in Figure 2. Excitation coils are installed above and below the measuring tube. A magnetic field is generated after the excitation current passes through the measuring tube. A pair of electrodes are mounted on the inner wall of the measuring tube to make contact with the liquid. The excitation current is provided by the converter.
3.Advantages The measurement channel of the electromagnetic flowmeter is a smooth straight tube without a flow resistance detection element. It is not easy to block and is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, mineral slurry, mud and sewage Wait.
The electromagnetic flowmeter does not generate pressure loss caused by the detection flow. The resistance of the meter is only the resistance along the pipeline of the same length, which has a significant energy saving effect. It is most suitable for large-diameter water supply pipelines that require low resistance loss.
The volume flow measured by the electromagnetic flowmeter is actually not significantly affected by changes in fluid density, viscosity, temperature, pressure, and conductivity (as long as it is above a certain threshold).
Compared with most other flow meters, the front straight pipe section has lower requirements.
The electromagnetic flowmeter has a large measurement range, usually 20: 1 to 50: 1, and a wide range of optional flow rates. Full-scale value liquid velocity can be selected within 0.5 ~ 10m / s. Some types of instruments can be expanded and reduced in accordance with the needs of the site (for example, a 4-digit potentiometer is used to set the meter constant). It does not need to be removed for offline actual flow calibration.
The caliber range of electromagnetic flowmeters is wider than other types of flowmeters, from a few millimeters to 3m. It can measure forward and reverse bidirectional flow, and it can also measure pulsating flow, as long as the pulsating frequency is much lower than the excitation frequency. The meter output is linear in nature.
Easy selection of fluid contact materials for corrosive fluids.
不能测量电导率很低的液体,如石油制品和有机溶剂等。 4. Disadvantages Electromagnetic flow meters cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. It is not possible to measure gases, vapors and liquids with large bubbles.
Due to the limitation of lining materials and electrical insulation materials, general-purpose electromagnetic flowmeters cannot be used for higher temperature liquids; some types of instruments are used for liquids below room temperature, and the insulation is damaged due to condensation (or frost) outside the measuring tube.
5. The general products and special instruments on the classification field can be classified from different angles.
For example, according to the excitation current, there are DC excitation, AC (power frequency or other frequency) excitation, low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation. The waveforms of several excitation methods are shown in Figure 3.
According to the system classification of output signal wiring and excitation (or power) wiring, there are four-wire system and two-wire system.
According to the converter and sensor assembly methods, there are separate and integrated types.
According to the connection method of flow sensor and pipeline, there are flange connection, flange clamp connection, sanitary connection and thread connection.
According to whether the flow sensor electrode is in contact with the measured liquid, there are contact type and non-contact type. According to the structure of the flow sensor, there are short tube type and insertion type.
Classified by purpose, there are general-purpose, explosion-proof, sanitary, water-proof and diving.